US 7606060 B2
An SRAM cell that is accessed by a single word line and separate access transistors for read and write operations. A pair of write bit line transfer devices provide respectively access to the right and left sides of cross coupled pull-up, pull-down transistor pairs for a write operation, and a single read bit line transistor in series with the word line transistor, when selected, reads the content of the cell.
1. A static random access memory comprising in combination:
a plurality of bi-stable memory cells arranged in rows and columns;
a pair of write bit lines common to each memory cell in a column;
a single pre-charged read bit line common to each memory cell in a column;
each cell including a pair of write data field effect transistors respectively connecting said each cell in column to said pair of write bit lines, a gate of each of said pair connected respectively to said pair of write bit lines for writing data into each cell;
a single read data field effect transistor connecting said each cell to said read bit line, with its gate coupled to a node of said each cell in a column for reading data from said cell;
a single field effect transistor connected to said pair of write data field effect transistors and said read data field effect transistor with its gate coupled to a single word line to control both read access and write access to said each cell.
2. A static random access memory as in
said pair of write data field effect transistors are each n-type field effect transistors;
said read data field effect transistor is an n-type field effect transistor, and
said single field effect transistor is an n-type field effect transistor.
This application contains subject matter that is related to the subject matter of the following applications, each of which is assigned to the same assignee as this application, International Business Machines Corporation of Armonk, N.Y. Each of the below listed applications is hereby incorporated herein by reference in its entirety. This application is a continuation of U.S. patent application Ser. No. 11/334,647 filed on Jan. 18, 2006, now U.S. Pat. No. 7,295,458 issued on Nov. 13, 2007, and is hereby incorporated herein by reference in its entirety.
IBM ® is a registered trademark of International Business Machines Corporation, Armonk, N.Y., U.S.A. S/390, Z900 and z990 and other names used herein may be registered trademarks, trademarks or product names of International Business Machines Corporation
1. Field of the Invention
This invention relates to an improved Static Random Access Memory (SCRAM) cell and more particularly to stable cell with only one word line.
2. Description of the Background
Typical prior art CMOS SRAM cells have a six-transistor typology, such as that shown in
An SRAM array has n rows and m columns of SRAM cells with cells of a row sharing a single word line WL and cells of a column sharing a pair of bit lines BLL and BLR. During standby, all of the word lines WL are low (i.e., at GND) and all bit lines are biased to the voltage level of the power supply Vdd. Thus, the pass-gate transistors of each cell are shut off. A cell state representing a 1 data bit, for example, is established with P1 and N2 on and P2 and N1 off so that the node L at the left of the cell is high (Vdd) and the node R at the right of the cell is low (GND). With P1 and N2 off and P2 and N1 on, node L is low and node R is high. During access time to the SRAM array a row is selected by driving its word line WL high to Vdd, which turns on half of the pass gates connected to that word line, namely, those connected to a high node. Thus, for each cell along a word line, one pass gate is turned on.
During a read access, a cell in a selected column, where its bit lines precharge devices are turned off, pulls down one of the bit lines (BL or BR) from its pre-charged high state (Vdd) by the “ON” passgate of the full selected cell (one that is selected in both the word and column dimension). A sense amplifier detects the differential voltage between the bit lines and generates an output. The cells along a selected word line likewise generate a differential voltage, but these signals are ignored. These cells along a selected word line that are not selected by the columns are commonly referred to as “half selected” cells. Unselected columns could have their bit line precharge devices either turned off or stayed on, depending on the design preference.
High performance designs, such as for cache where read and write operations are performed within a short clock cycle, cell beta ratios that are typically in a range between 2.2 and 3.5 in order to avoid unintended changes in the state of a cell due to a so called “half-selected cell disturbance”. The beta ratio is a function of the ratio of the channel resistance of the pass gate transistor to the channel resistance of the pull down transistor. Half-selected cell disturbance occurs when a word line connected to a memory cell of unselected columns with bit lines biased to a predetermined Vdd is activated. This causes the body voltage of a pass-gate transistor of a non-selected cell to be more conductive than the cell pull down transistor, there by causing the ground state node (“0”node) to switch states and disrupt the state of the memory cell.
In fast switching environments a high beta ratio improves the margin preventing half-selected cell disturbances. However a higher-beta ratio results in decreased cell performance, such as lower write. Particularly, sub 100 nm silicon technologies are having increased problems with cell SRAM cell stability when trying to write to the cells at high rates due to the inherent variations in individual cell beta ratios as a result to limitations in the degree to which device channel parameters can be controlled at these extremely small devices sizes.
Adding additional transistors to the basic six-transistor SRAM cell have been proposed in the prior art in order to prevent half-selected cell disturbance by providing separate access paths for reading and writing. However these prior art proposals have not proven satisfactory. Particularly, as they employ two separate word lines, one for writing and one for reading.
An object of this invention is the provision of an improved SRAM cell that provides cell stability, with reduced cell area requirements without impacting the writing speed to the cell.
Another object of the invention is the provision of an improved SRAM cell with separate cell access paths for reading and writing and only a single word line. Briefly, this invention contemplates the provision of an SRAM cell that is accessed by a single word line and separate access transistors for read and write operations. A pair of write bit line transfer devices provide respectively access to the right and left sides of cross coupled pull-up, pull-down transistor pairs for a write operation, and a single read bit line transistor in series with the word line transistor, when selected, reads the content of the cell.
The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed taken in conjunction with the accompanying drawings in which:
The detailed description explains the preferred embodiments of the invention, together with advantages and features, by way of example with reference to the drawings.
Referring now to
Read access to the cell nodes L and R is provided by a read access NFET 32 whose drain is connected to ground via the word line NFET 28 and whose source is connected to a read bit line rbl, which is common to all the cells in a column and is coupled to control logic and a sense amplifier, not shown. The gate of NFET 32 is connected to the node L in this exemplary embodiment.
In operation, in order to write to the cell, one of the normally low write bit lines wbl_l or wbl_r is brought high while the other write bit line remains in its normally low state. Concurrently, the word line wl is brought high. The node (L or R) connected to write access NFET (24 or 26) whose gate is high is therefore connected to ground through that write access NFET and the word line NFET 28. If the node is in a high state, it is pulled down, causing the cell to change states. If the node is in a low state, the cell maintains its existing state.
Similarly, in order to read the state of the cell, the read bit line rl is precharged high before a read operation. Concurrently, the word line wl is brought high so that the drain of read access transistor 32 is connected to ground via word line NFET 28. In this exemplary embodiment, with the gate of read access NFET 32 connected to the left node L, if the node L is in its high state, NFET 32 conducts, connecting the read bit line rl to ground via NFET 28, thus providing a signal indicating the high state of that side of the cell. If node L is in a low state. NFET 32 does not conduct, and bit line rl remains in its high state, thus providing a signal indicating the low state of that side of the cell.
While the preferred embodiment to the invention has been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.